- completely redesign number converter UI with modern styling and dark mode support

- add bidirectional conversion (other bases to decimal)
- implement step-by-step conversion explanations
- add custom base support (up to base-36)
- add digit grouping options with different separators
- improve input validation with error messages
- add copy-to-clipboard functionality
- enhance accessibility features
- implement local storage for user preferences

Author: TobiasMue91

PROMPTS.md

@@ -243,6 +243,7 @@ Our goal is to improve it thoroughly.
 * The tool should stay in the scope of this single HTML page.
 * Provide the updated code with a minimal amount of line breaks and indentations in order to save tokens.
 * Implement modern, well-structured and fitting design, UI and UX.
+* Take your time thinking and planning. Don't provide the finalized code before creating a watertight plan.
 </requirements>
 
 <task>